The present invention relates to satellite communications and more particularly relates to a synchronization method for a processing communication satellite system.
Processing communication satellites routinely relay vast amounts of data between multiple earth terminals each day. Typically, the data is transmitted between the earth terminal and the satellite in bursts. Each burst is composed of discrete cells of information, which collectively form frames. The frames are composed of a predetermined number of symbols, and each symbol may represent multiple bits of information. Frames typically contain specific header information, including, for example, identification, routing, or error detection and correction coding. Additional information may also be used to identify the beginning and ending of each frame within the uplink or downlink transmission.
Processing communication satellite systems typically use single carrier time division multiplexing, or TDM, on the downlink and time division multiple access, or TDMA, on the uplink. On the uplink, the earth terminals are allocated time slots in which they independently transmit bursts to the satellite. Each earth terminal is required to keep its timing system aligned with that of the satellite so that its transmissions do not overlap with the transmissions sent by other earth terminals. One problem faced by existing TDMA systems is the synchronization of the bursts sent by the earth terminals on the uplink to ensure that the bursts do not overlap at the satellite. As a minimum, the uplink bursts must be timed to arrive at a specific time (or specific uplink symbol number) within an uplink frame to avoid overlap from bursts from other earth terminals. Ideally, the time of arrival of a burst would be so precise that its symbols align exactly with the symbol epochs of the satellite receivers, so that the burst may be demodulated without overt symbol timing tracking processing.
The uplink signals and downlink signals are subject to time delays or latencies corresponding to the time it takes the signal to travel between the earth terminal and the satellite. The signals from the earth terminal must be launched in a timely manner in order to arrive at the satellite in the correct time slot at the correct symbol time, and therefore, the time required for the bursts to travel between the earth terminal and the satellite must be taken into account.
Because of the potential for overlapping transmissions, bursts are typically surrounded by guard bands. Guard bands are time intervals in which no transmissions occur, and they are included in transmissions in order to provide a margin of safety against timing error and inter-symbol interference. There are several disadvantages associated with the use of guard bands in transmissions, including wasted bandwidth and reduced data transmission rates. In the past, however, guard bands were required because of the lack of precision in synchronization methods.
Furthermore, in the past, there were no effective synchronization methods which allowed the synchronization of uplinks and downlinks operating at different speeds. Because this is precisely the typical situation in present processing satellite communications, the lack of synchronization methods posed a significant problem. Thus, the satellite communication industry has had to make do with sub-optimal synchronization techniques. For example, as a result of sub-optimal synchronization, prior systems were forced to operate with unacceptably large errors in timing, guard bands, and complex and power consuming demodulation or symbol timing recovery hardware.
Thus, a need has long existed in the industry for a method of precisely synchronizing the uplink and downlink of a satellite communication system.